The present invention refers to the field of refrigerating or chilling systems of the so-called xe2x80x9cfree-coolingxe2x80x9d type.
Refrigerators or chillers with free-cooling are currently available on the market and are generally used for technological sites (data banks, telephone exchanges, etc.). There follows a brief explanation with reference to FIG. 1, which shows a currently known typical free-cooling system. The system is designated as a whole by reference number 1 and comprises a primary circuit 10, a secondary or user""s circuit 20, and a refrigerating or cooling circuit 30. The refrigerating circuit comprises a compressor 31, a condenser or condenser battery C, an expansion valve 34, and an evaporator E. It further comprises a line 32 between the compressor and the condenser, a line 33 between the condenser and the expansion valve, a line 35 between the expansion valve and the evaporator, and a line 36 between the evaporator and the compressor, all these being indicated in the figures with dash lines.
The secondary circuit 20 generally comprises a disconnector line referenced 21, a delivery line 22 with pump P2; a number of users"" appliances or terminals referenced U, Uxe2x80x2, each on a respective user""s line 23, 23xe2x80x2, the lines 23, 23xe2x80x2 etc. being generally connected in parallel, and each having a bypass line 25, 25xe2x80x2; and a return line 26.
The primary circuit 10 comprises a free-cooling battery EC, a delivery line 12 at outlet from the evaporator, a return line 13 with pump P1, a bypass line 14 for bypassing the free-cooling battery, said line extending to a three-way valve referenced V, a line 15 extending to the free-cooling battery FC, a line 16 extending between the free-cooling battery FC and the three-way valve, and a line 18 extending between the three-way valve and the evaporator.
The free-cooling battery FC is a finned-tube battery. In the tubes thereof a fluid of the primary circuit (generally water) circulates. Air circulates around the tubes, so as to obtain, if the air temperature allows, a xe2x80x9cfreexe2x80x9d cooling of water. The free-cooling battery FC is generally set upstream of the condenser, with respect to the air flow.
The assembly shown in the box of FIG. 1 and referenced 50 is generally supplied as a single or self-contained apparatus called xe2x80x9crefrigerator or chiller with free coolingxe2x80x9d or xe2x80x9cfree-cooling chillerxe2x80x9d intended for being connected to the user""s circuit.
Free cooling chillers are able to exploit the low temperature of outdoor air for cooling water to be sent to a user""s system or secondary circuit 20 and are used in systems that require cooling energy also at low temperatures, as in the case of technological systems. They differ from normal chillers in that the finned battery FC is provided, which operates as an air-water heat exchanger, and is located upstream of the condenser battery C, of the refrigerating circuit 30. Air moved by fans traverses in series, first, the air-water battery FC, and then, the condenser C of the refrigerating circuit.
The purpose of the additional battery FC is to take advantage of a low air temperature for cooling the return water coming from the system before sending it to the evaporator of the machine. In this way, a free cooling is obtained which leads to a saving in terms of electrical energy, in that less compressor work is required.
Free-cooling chillers have, therefore, two different operating regimes: normal operation and free-cooling operation.
Switching from normal operation to free-cooling operation is controlled by a microprocessor control system (not shown): when air temperature at the batteries inlet is lower than water temperature at the unit inlet, the free-cooling system is activated.
Under normal operating conditions, the valve V has the way to the line 14 open and the way to the line 16 closed : the free-cooling battery FC is therefore bypassed or excluded. As soon as air temperature, measured by the probe TA, drops below the return water temperature, measured by probe TW2, the valve V opens the way to the line 16 and closes the way to the line 14. In such a way, the return water is cooled by outdoor air in the additional battery FC before entering the evaporator.
In this way, the consumption of electricity by the compressors is reduced. The purpose of the refrigerator or chiller is to produce refrigerated water at a desired temperature, measured by the probe TW1. Obviously, if water is pre-cooled by the free-cooling battery, the amount of refrigerating energy to be supplied, by means of the compressors, to the evaporator decreases, with consequent reduction in the consumption of electricity.
Free-cooling is said to be partial when water is cooled in part freely by the exchange battery and in part in the evaporator, thanks to the operation of the compressor/s; it is said to be total when the entire refrigerating load is supplied freely by the exchange battery.
The percentage of free-cooling as compared to the total refrigerating load required depends upon outdoor air temperature, upon the refrigerating load required from the system, upon refrigerated water temperature desired at outlet from the refrigerator, and upon water inlet temperature in the free-cooling battery.
FIG. 2 shows, as a function of outdoor air temperature, how the load is divided between the free-cooling battery and the compressors in the case of power (capacity) linearly decreasing with external temperature: 100% at 35xc2x0 C., 40% at 5xc2x0 C. The temperature at the delivery side to the system, measured by the probe TW1, is 10xc2x0 C. In the diagram of FIG. 2, the grey area indicates the power (capacity) from the free-cooling battery.
As may be seen, when outdoor air temperature drops below 13xc2x0 C., the free-cooling battery starts to supply part of the power required by the system. The entire power is supplied by the free-cooling battery for temperatures below 7xc2x0 C.
The system described has constant flow rate.
The user""s terminals or batteries U, Uxe2x80x2 in fact, are controlled by three-way valves VU, VUxe2x80x2. At full load, all the water passes through the user""s batteries U, Uxe2x80x2 whilst, as the required power is reduced, an increasingly greater part of the water flow bypasses the user""s batteries through the lines 25, 25xe2x80x2. Downstream of the valves VU, VUxe2x80x2 however, the flow rate remains constant whatever the load required by the system.
Also known are systems in which the user""s terminals U, Uxe2x80x2 of the system may be controlled with two-way valves which directly choke the flow of water to the user""s batteries U. Uxe2x80x2. The pump P2 varies the number of revolutions to adapt to the new flow rate of the system. The secondary circuit thus operates with variable flow rate. Systems with variable flow rate are becoming increasingly common because they enable a considerable saving on the pumping expenses and because the cost of regulators or controllers with inverter for the pumps is markedly decreasing.
In known systems the flow rate variation, however, must be limited to the secondary or user""s circuit alone and cannot take place in the primary circuit 10, a portion of which passes through the evaporator. The primary circuit, in fact, cannot undergo flow rate variations in operation, because a flow rate variation through the evaporator would lead to failure of the compressor 31. In known systems, it is therefore not possible to use a free-cooling battery with variable flow rate.
In systems with constant flow rate the return temperature measured by probe TW2 of FIG. 1 is directly proportional to the load required by the system. For example, if water leaves the chiller assembly 50 at 10xc2x0 C., at 100% of the load it returns at 15xc2x0 C. At 75% of the load, the return temperature drops to 13.7xc2x0 C.; at 50% it becomes 12.5xc2x0 C.; at 25% it becomes 11.3xc2x0 C.; and at zero load, it becomes equal to outlet temperature, i.e.,
The situation is different in the case of a system with variable flow rate in the secondary circuit. The yield (power output) of a user""s battery or terminal decreases at a clearly lower rate in percentage terms with respect to the flow of refrigerated water that passes through it. As an immediate consequence of this, the thermal head (difference in temperature) of water between inlet to and outlet from the user""s battery or terminal increases as the flow rate decreases.
In a system with variable flow rate, the thermal head increases continuously as the load decreases, and the system behaves in a manner opposite to that of the system with constant flow rate.
The consequences on the dynamics of the temperatures of the system are immediately deducible. In fact, whilst in the case of a system with constant flow rate the return temperatures decrease as the load decreases, in the case of a system with variable flow rate the said temperatures increase. At 75% of the load, the return temperature becomes 19.3xc2x0 C. as against the 13.7xc2x0 C. mentioned previously. At 50% of the load, the return temperature becomes 23.1xc2x0 C. as against the 12.5xc2x0 C. of the system with constant flow rate. At 25% of the load, the return temperature becomes 26.3xc2x0 C. as against 11.3xc2x0 C. of the system with constant flow rate.
If it were possible to operate the free-cooling battery at a variable flow rate, the advantages would be considerable because this would involve a greater exploitation of the free-cooling battery.
The purpose of the present patent application is therefore, in a free-cooling refrigerating system, to enable operation with variable flow rate also in the part of the primary circuit relating to the free-cooling battery, thus exploiting the possibilities of the free-cooling battery, in the best possible way.
In other words, a new refrigerating unit comprises a traditional refrigerating circuit and a primary free-cooling circuit which has, between the delivery or outlet line from the evaporator, and the entry or inlet line to the evaporator, a bypass line with a storage tank. Preferably, the pump of the primary circuit is mounted on the outlet or delivery line from the evaporator.
When mounted in a system with user""s appliances requiring a variable flow rate, the new chilling unit enables a variable flow rate not only in the user""s circuit but also in the part of the primary circuit that passes through the free-cooling battery, albeit always having a constant flow rate through the evaporator, as the flow rate through the evaporator is at any moment integrated by means of the storage tank.
The new refrigerating/chilling unit makes it possible to use the free-cooling battery at variable flow rate with all the inherent advantages, without, however, this adversely affecting the life of the refrigerating circuit, and in particular of the compressor or compressors of the latter.